Trespasser, gate-crasher (8)
I believe the answer is:
intruder
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">trespasser</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">gate-crasher</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for intruder that I've seen before include "One entering unasked" , "Butter in" , "Unwelcome sort" , "Illegal entrant" , "One entering uninvited" .)